(i) Field of the Invention
This invention relates to a monitor assembly for monitoring any possible corrosion and/or the corrosion protection and potential of a carbon steel vessel containing a caustic liquor. More particularly it is directed to such a monitor assembly for use in carbon steel vessels used in the wood pulp industry which contain pulping liquor and which could be protected from corrosion by anodic protection.
(ii) Description of Prior Art
Carbon steel is known to be generally resistant to corrosion in caustic solutions due to build-up thereon of passive films. However, such passive films are susceptible to localized breakdown or to uniform corrosion, depending on the electrochemical potential exerted on the carbon steel by the caustic solution. Kraft or soda-process pulping liquors generally contain oxidants, e.g. polysulphides and organic species, which influence the corrosion potential of a carbon steel vessel and can cause localized corrosion in the form of caustic stress corrosion cracking or uniform corrosion.
Similar forms of corrosion have also been observed in the Bayer process which is used in the aluminum industry to refine Bauxite ore into a purer form of alumina.
The corrosion environment in vessels used in the pulp industry, e.g. kraft or soda pulp digesters and white liquor clarifiers, is alkaline with concentrations of NaOH in the range 8 to 100 g/L, Na.sub.2 S up to 40 g/L and Na.sub.2 CO.sub.3 around 25 g/L. The common alloys of construction are carbon steels like A516 grade 70 or A285 type C. In a number of cases pulp digesters made from these steels have suffered from caustic cracking or from high rates of uniform corrosion, and have required repair or reconstruction at considerable cost.
One procedure heretofore used to control corrosion in such digesters, as well as in Bayer-process vessels, has been anodic protection. Many patents teach the concept of corrosion control by inducing passivity in the metal by anodic polarization techniques. With such a technique, the vessel to be protected against corrosion by a chemical contained therein is anodically polarized with respect to an inert cathode suspended in the corrosive liquid in the vessel. An electric current is passed between the metallic vessel and the inert cathode so as to maintain the electrical potential of the vessel in the passive region. The necessary electrical potential can be determined by means of an anodic polarization curve, or by controlled potential immersion testing, or by controlled potential stress corrosion testing. The passive region can be identified after such tests, thus providing data indicative of the potential range within which the vessel should be maintained in order to attempt to minimize corrosion.
Amongst the prior patents dealing with this technique are: Mueller and Watson, U.S. Pat. No. 3,009,865, Banks et al., U.S. Pat. Nos. 3,371,023 issued Feb. 7, 1968, 3,375,183 issued Mar. 26, 1968, 3,378,472 issued Apr. 16, 1968, 3,379,629 issued Apr. 23, 1968, and 3,409,526 issued Nov. 5, 1968; Hoey, U.S. Pat. No. 3,442,779 issued May 6, 1969; and Hulthe, U.S. Pat. No. 4,036,716 issued July 19, 1977.
In order to carry out constant-potential anodic protection successfully, a means of measuring the vessel potential is essential and a reference electrode is used for this purpose. Many common reference electrodes, e.g. the silver/silver chloride electrode, are unsuitable for potential measurement in pulping liquors because they are poisoned by contact with reduced sulphur species, e.g. sulphide. However, it is well known that metallic silver, molybdenum and other metals can perform as a reproducible reference electrode in pulping liquors as taught by Every and Banks in U.S. Pat. No. 3,462,353, even though that patent more preferably teaches the use of metal/metal salt electrodes.
U.S. Pat. No. 4,285,232 by Garner, A. issued Aug. 25, 1981 teaches a method for monitoring the effectiveness of electrochemical protection by means of a monitor assembly which carries two corrosion coupons. The assembly is designed so that one coupon is protected, and the other is unprotected but otherwise identically exposed.
The electrochemical potential of a large carbon steel vessel, e.g. a pulp digester, is not uniform throughout the vessel but can vary substantially from place to place. Even when anodic protection is applied to such a vessel, the potential can be substantially non-uniform. The potential can vary because it is determined by three factors, each of which vary with position in the vessel. These factors are (a) liquor composition (b) the nature of film or deposit on the steel surface and (c) the proximity of the cathode in an anodic protection system. Because the potential is position-dependent, it is essential to monitor the potential at the same location as a monitor assembly which is used to monitor the effectiveness of protection. Potential monitoring and corrosion monitoring could be carried out using two separate probes one of which carries a reference electrode, and the other carries a corrosion monitor assembly. However, it is highly undesirable to locate two separate probe parts in close proximity on a carbon steel vessel, particularly in the case of a digester which is a pressure vessel and subject to strict codes and regulations. Moreover, it has not previously been proposed to mount the reference electrode on the same probe which carries the monitor assembly because of the difficulties of ensuring that only the reference electrode is wetted by the process lliquor, while the electrically connectable mount rod be maintained substantially dry.